Conventional friction devices not infrequently have the problem of premature failure. Most friction devices operate with constant friction torque and/or constant damping over the entire torsion damper characteristic or are adjusted to a range of characteristics or a spring level. For this purpose, control plates engage in the spring windows or at least have control edges for the springs, e.g., U.S. Pat. No. 7,559,844. However, the level of required friction torque is determined only by the required damping of the powertrain resonance in the respective gear of the transmission. The higher the resonance, the higher the required damping (i.e., friction torque) and, therefore also, the higher the loading of the friction device. The load manifests itself conventionally in higher forces (usually disk springs) and, therefore, higher area pressures in the contact points of the friction device.
In a friction device with a stepped friction torque characteristic such as in U.S. Pat. No. 7,559,844, for example, a distinctly abrupt torque increase which is noticeable as wear at the friction device occurs at the transition between two friction torque levels.
One solution could consist in configuring the preloading force of the aforementioned disk spring of the friction device to be variable over the swivel angle. Reference is made to U.S. Pat. No. 4,583,959, for example. However, this kind of construction principle is itself subject to significant wear and has an internal friction torque which must be taken into account in the configuration of the friction device in its entirety.
It is thus an object of the present invention to minimize the negative effects of the discontinuous torque curve.